Honestly, any helpful advice, and sincere discussion of topics builds credibility. Then when people get to know you then you can be confident what you post will be seen in the light it is meant to be.
Often we have a new poster who comes in and is nervous about fitting in.. this leads to them posting their "best" material, or even exaggerated claims. We ignore most of that and try to get to know them so they'll feel more comfortable, but sometimes a new person will try too hard and come off as a complete whack, or a fake. It's unfortunate, but it happens.
Interestingly enough, however, we've had some people that came off terribly (or were even fakes to start with) that stuck around and became legitimate heroes with good reputations.